Light-emitting devices based on organic light-emitting diodes (OLEDs), also known as organic electroluminescent (EL) devices, are an emerging display technology. In essence an OLED comprises a thin organic layer or stack of organic layers sandwiched between two electrodes, such that when a voltage is applied visible or other light is emitted. At least one of the electrodes must be transparent to light. For display applications the light must of course be visible to the eye, and therefore at least one of the electrodes must be transparent to visible light.
There are two principal techniques that can be used to deposit the organic layers in an OLED: thermal evaporation and solution processing. Solution processing has the potential to be the lower cost technique due to its potentially greater throughput and ability to handle large substrate sizes. Significant work has been undertaken to develop appropriate materials, particularly polymers. More recently dendrimers that are photoluminescent in the solid state have been shown to have great promise as solution processible light-emitting materials in OLEDs (S-C. Lo, et al Adv. Mater., 2002, 13, 975; J. P. J., Markham, et al Appl. Phys. Lett., 2002, 80, 2645).
Dendrimers are branched macromolecules with a core and attached dendrons. Dendrons are branched structures comprising branching units and optionally linking units. The generation of a dendron is defined by the number of sets of branching points; see FIG. 1. Dendrons with the same structure (architecture) but a higher generation, or order, are composed of the same structural units (branching and linking units) but have an additional level of branching. There can be surface groups on the periphery of the dendrons.
Light-emitting dendrimers typically have a luminescent core and in many cases at least partially conjugated dendrons. Further examples of light-emitting dendrimers include those found in P. W. Wang, et al Adv. Mater., 1996, 8, 237; M. Halim, et al Adv. Mater., 1999, 11, 371; A. W. Freeman, et al J. Am. Chem. Soc., 2000, 122, 12385; A. Adronov, et al Chem. Comm., 2000, 1701.; C. C. Kwok, et al Macromolecules, 2001, 34, 6821. Light-emitting dendrimers have the advantage over light-emitting polymers in that the light-emitting properties and the processing properties can be independently optimised as the nature of the core, dendrons and surface groups can be independently altered. For example, with dendrimers that contain light-emitting cores, the emission colour of a dendrimer can be changed by simply changing the core. The nature and generation of the dendrons in a dendrimer have been shown to influence the charge transporting and processing properties (J. M. Lupton, et al Phys. Rev. B, 2001, 63, 5206). Other physical properties, such as viscosity, may also make dendrimers more easily tailored to the available manufacturing processes than polymers.
Dendrimers have previously been used in OLED applications as a single component in a film (i.e. a neat film) or in a mixture with a molecular material (Markham et al, loc cit) or in a mixture of more than one dendrimer of different type (i.e. different cores), e.g. J. M. Lupton et al. Adv. Funct. Mater., 2001, 11, 287. In this later case one type of organic dendrimer was used as a host for a guest organometallic dendrimer but the efficiency of the device was low.
Although progress has been made in the development of solution processible OLEDs there is still the need for OLEDs with improved efficiency and lifetime.
The current invention is directed to the production of mixed dendrimer films and OLEDs containing them that solve some of the problems in the prior art.